"My favorite thing is to stay in my pajamas all day watching movies. In fact, it's very strange for me to be wearing a dress and shoes at this hour. I'd usually still be in jammies," says the actress, who's hot-footing it all over Tinseltown doing press interviews. It's 11:30 on a Friday morning, and the statuesque Theron has been up since 5. Showing no signs of wear and tear, however, she's ever-perky, revealing a grounded woman proud that what's mainly responsible for her shimmery ascent is the will to tough things out.

"It's not like I just went and got the movies I've done. It was a struggle pretty much all the time. It still is. I have to find movies that will challenge me, and then I need to go in and prove myself," she says.

Reared on a farm in her native Benoni, South Africa, Theron, just six years in Hollywood, has already proven herself a standout in a succession of features, including "The Devil's Advocate," opposite Al Pacino and Keanu Reeves, and "Cider House Rules," (as an "older woman" who introduces Tobey Maguire's sheltered orphan character to sex and romance).

In "Reindeer Games," directed by John Frankenheimer, ("Manchurian Candidate," "Ronin") Theron is a sales clerk hungry to start life with an ex-con, portrayed by Affleck. Forced into a scheme to rob a gambling casino on Christmas Eve, the couple's mission then becomes figuring out how to stay alive.

"I related to Ashley in the sense that she's just somebody who wants to find true love, to belong somewhere. But she really goes to desperate measures for it," says Theron, whose nickname is Charlie. On the "Reindeer" set, she was dubbed Sparks and Sparky.

"John [Frankenheimer] started calling me Sparks one day, and then Ben did too. For the longest time I didn't know why. So I asked Ben, `Where did John come up with the whole Sparks thing?'

"He said, `Because you're annoying -- you talk too much. You're like one of those World War II radio reporters: `Here's Sparks McGrath, and I'm gonna bring you the latest right now!' You just don't stop babbling!' Ben was always on my back with that. He's got the greatest sense of humor. He's like the big brother I never had who was constantly pulling my pigtails."

Last summer Theron co-starred with Johnny Depp in the thriller "The Astronaut's Wife." The film drew tepid box office returns following mixed reviews. But Theron's performance was critically praised.

The actress takes a laissez-faire attitude about monitoring the dollars her movies bring in. "It's not something that drives me and makes me get out of bed in the morning," she says, claiming to act "for a different reason: It's like air to me. It makes me feel alive. It satisfies me completely."

Sometimes, slipping into the back row, she audits drama workshops. "I love workshops celebrating acting as something that's under our skin -- and that doesn't make it fake. I like to watch actors struggle to find the right emotion, fail, try again and then find it. . . . I've learned a lot from it."

After starring in a string of period pictures, including "That Thing You Do!" (1950s), "Cider House Rules" ('40s) and both the upcoming "Navy Diver" ('50s, '60s), as Robert De Niro's alcoholic wife, and "The Legend of Bagger Vance" ('20s, '30s), Theron enthuses: "There's more magic to period films, because it's something you don't know that well.."

Playing "make-believe" is what drove Theron to act in the first place. After studying ballet for years, she saw her hopes for a dance career dashed by a serious knee injury suffered in a Joffrey class. "Dancing is performing, and I wasn't doing that anymore. I felt like I had this big hole in my soul," she recalls. "I wanted to perform -- to put on makeup and costumes and tell stories."

She'd been a professional model from age 14. "It was my waitressing job -- my income," she explains. Just two weeks after arriving in Los Angeles to seek screen work, Theron acquired an agent, who spotted her in line at a Hollywood Boulevard bank. Within eight months, she'd won her first part. More roles came right away: "That Thing You Do!" "Mighty Joe Young," Woody Allen's "Celebrity." Now she may co-star as singing-dancing Roxie Hart in the possible film version of the Broadway musical "Chicago."

Born to a German mother and French father, Theron grew up, she says, "50 percent tomboy, 50 percent princess." On the farm, she faithfully attended to chores -- milking the cow, feeding sheep -- and in the time left over, let her imagination soar. "We had almost no television and no computers. So I really had to create things to try to entertain myself," the only child recalls.